Causes of & Reasons for Small Breasts


Breasts come in all shapes and sizes. Some are round, some are pointed, some are asymmetrical. Some are large and some are small. There is absolutely nothing wrong with your breasts or nipples based on their size or shape.

Some of us can self-perceive that we have underdeveloped breasts if we note a discrepancy between our body image and the internalized images of socially acceptable or desirable breast size and shape. It’s worth noting that these images can come from less than reliable sources, embedding an unrealistic standard of “beauty” in our minds.

How Is Breast Size Measured?

Before we can really talk about breast size, we should first mention how it’s measured. Breast size is usually stated in terms of the size of your bra. 

For a deeper dive on measuring your bra size, click here

Breast size usually focuses on the cup size of your breasts. However, it is worth noting that breast size can fluctuate over a woman’s lifetime, and especially during pregnancy.

What Are Underdeveloped Breasts Anyway?

“Small” is a pretty relative term and it may not mean the same thing to different people. If I’m a F or G cup, I might consider C cups small. If you’re a C, you might think an A cup is small. 

The average bra size in the U.S. is 34DD. However, of course, there are many women significantly smaller, and larger than this size (it is after all a mere average).

Not to mention that the same cup size might look smaller and bigger depending on your frame and the shape of your breasts. So “small breast size” needs to be better explained and qualified.

One misconception about small breasts that is worth clearing up: You’re neither more nor less capable of nursing with large breasts or small breasts. Breast size doesn’t indicate how much breast milk you can produce and large breasts aren't automatically better for the job.

What Determines Breast Size?

Okay, now you know your breast size, you might wonder WHY? Why is one person a 36A and another a 32E? In a way, this is like wondering why we don’t all have the same size and shape noses… because we’re all different! 

So what causes such extreme differences in bust size? What causes “small breasts”? The answer is really in your genes. Small breasts or even absent breast development can sometimes be caused by underlying genetic conditions.


Yep, you can attribute both the size and shape of your breasts and even the composition of your breast tissue, mostly to your genes.

According to Nature, “Breast size is a highly heritable trait. A twin study previously estimated the heritability of bra cup size to be 56%8. Several genome-wide association studies have also identified common genetic variants associated with breast size.”


Another major factor when it comes to breast size is body weight. Breasts are complex body parts but they are composed significantly of adipose fat. This may mean that individuals with higher body fat will often have larger breasts. Of course, weight is determined by both genetics and lifestyle.

Is Small Breast Size a Medical Concern?

Hypomastiais a medical term describing underdeveloped breasts (also called breast hypoplasia, or mammary hypoplasia). 

Hypomastia is defined as breast size of 200 mg (mL) or less in an adult female, represents a significant decrease in size of breast tissue relative to standard breast size adjusted for age and developmental status (The Breast (Fifth Edition), 2018).

Some congenital causes include ulnar–mammary syndrome (a rare inherited disorder), Poland syndrome, Turner syndrome, and Hormonal Imbalance.

If this is the case your primary care doctor may refer you to a gynecologist (a specialist who focuses on reproductive concerns, including reproductive hormones) or an endocrinologist (a specialist who focuses on all hormonal problems).

Poland syndrome is a disorder in which according to the NIH, “People are typically missing part of one of the major chest muscles, called the pectoralis major." There can also be other associated abnormalities that affect the hands and spine.

What Can Cause Breast Size to Change?

While breast size is mostly determined by your genes, that doesn’t mean it stays the same throughout your life. Indeed, most women experience many changes in their breasts over the course of their lifetime. Here are some of the key causes of changes to your breast size (and shape).

1. Age: Puberty to Menopause

From puberty through adolescence, peak reproductive years, and into menopause, our bodies go through many changes. These developmental stages are often guided by hormonal changes which can definitely affect your breasts. Naturally, you'll start with smaller breasts, which will grow until you're fully developed.

2. Menstrual Cycle

Hormones also play a major role in your menstrual cycle and these can cause monthly changes to your breast size. Many women especially notice fluid retention, breast swelling, and tenderness just before their period as part of their premenstrual symptoms (PMS) and/or during ovulation, around the middle of their cycle.

3. Hormonal Birth Control

Hormonal birth control is used to prevent unwanted pregnancy but the hormones in play can cause fluid retention and breast swelling in some women. These effects are often stronger in the first few months of taking a new birth control regimen.

4. Pregnancy

During pregnancy, your breasts may double or triple in size. According to the Mayo Clinic,

“One of the first signs of pregnancy is an increase in breast size. As early as two weeks after conception, your breasts start to grow and change in preparation for producing milk. Stimulated by the hormones estrogen and progesterone, the milk-producing glands inside your breasts get bigger and fatty tissue increases slightly.

By the end of your first trimester, your breasts and nipples will be noticeably larger, and they may keep growing throughout your pregnancy. Breast enlargement can account for at least a pound of the weight you gain while pregnant.”

Your breasts may remain enlarged for a while after birth.

5. Weight Gain or Loss

Weight fluctuation is one of the many reasons it’s recommended you go for a bra fitting regularly (every year or so). This is because the fat component of a woman’s breasts may make it a body area she’ll easily lose or gain weight.

However, it’s worth noting that it is difficult to target fat gain or loss to a specific body area. So if you lose or gain weight it will be distributed throughout your body, affecting other areas as well as breasts.

6. Smoking

Among the many health risks associated with smoking is a loss in skin elasticity, which can affect the delicate skin and breast tissue. This may cause breasts to sag. Some women perceive this as a change in size, though it’s more related to shape unless accompanied by other changes.

7. Breast Augmentation

Of course, one of the more obvious ways to change breast size is to go under the knife. Whether it’s breast augmentation (breast enlargement or breast enhancement) or reduction, this will cause a more extreme and often permanent change to your breast size. If you are considering breast augmentation, please seek out medical advice so you fully understand the procedure, pros, and cons.

When to See a Doctor

As we’ve outlined, breasts can change a lot throughout your lifetime and even your monthly cycle. These changes, like your monthly menstrual cycle, should become familiar to you over time. For example, you might learn to expect breast tenderness at a certain time of your cycle. 

However, some breast changes are worth reacting to. If you experience any of the following, it’s worth booking an appointment with your doctor:

  • Any lumps on your breasts, or around your armpit
  • Redness, rash, warmth, or swelling of breasts
  • Any pain in your breasts
  • Changes in the shape, color, or size of nipples that are not otherwise explained 
  • Itchy nipples or any discharge from your nipples

At the appropriate age, your doctor will also recommend regular mammograms to screen for breast cancer, in addition to the manual exams they usually perform at your annual checkup. 

Per the American College of Obstetricians and Gynecologists, "for women at average risk of breast cancer, screening mammography is recommended every 1–2 years beginning at age 40 years. If you have not started screening in your 40s, you should start having mammography no later than age 50 years."

Conclusion: There’s No “Right Size” When it Comes to Breasts

We are all different. We come in different heights, sizes. Some of us have big noses, others have soft chins. Some of us have ears that protrude, others have big feet. And, of course, some of us have large breasts and others have small ones.

We live in a time when we’re all trying to be more body-positive. To accept there’s beauty in our diversity and that there’s no single standard of beauty we all have to adhere to.

However, sometimes this is easier to say than to feel. There’s still tremendous pressure on women to look a certain way. This can come from media, the fashion industry, even (unfortunately) our friends and partners. So it’s not surprising that many of us are self-conscious about small breasts.

But feeling that way about small breasts doesn’t mean there’s something ‘wrong’. If you’re really struggling with your breast size or worried there might be an underlying medical cause, talk to your healthcare provider. But in general, this may be less about medical concerns and more about self-acceptance and love.

Medically reviewed by Dr. Chimsom T. Oleka, M.D, Written by Jane Flanagan — Updated on September 20, 2021.